The present invention relates to a method of controlling the titre of the exhaust gases in an internal combustion engine.
As is known, control systems for the internal combustion engines currently commercially available make use of strategies which, by controlling the air-fuel ratio (A/F) of the mixture supplied to the engine, make it possible to minimise emissions of pollutant agents. Efficient removal of these pollutant agents depends in practice on the composition of the exhaust gases of the engine.
According to the strategies commonly used, a concentration signal supplied by an oxygen sensor, representative of the A/F ratio or, equivalently, of the exhaust titre of the engine, is used to determine the quantity of fuel to be supplied to this engine in order to obtain a desired titre. On the basis of the curve of the concentration signal and as a function of the operating conditions of the engine, a controller of proportional-integral type calculates a correction coefficient in order to modify a nominal quantity of fuel in order to make the mixture supplied to the engine richer or leaner. By selecting appropriate values of a plurality of integration and delay parameters of the proportional-integral controller, it is possible to cause the correction coefficient and therefore the exhaust titre to oscillate with a predetermined frequency about mean values having a desired curve.
The values of the integration and delay parameters, corresponding to different operating conditions and desired titre values, are normally determined empirically during the calibration stage and stored in large-dimension tables.
This is a drawback as in the known strategies the number of parameters used to calculate the correction coefficient is high and therefore requires considerable and expensive non-volatile memory space.
The known strategies make it possible, moreover, to act alternatively either solely on the integration parameters or solely on the delay parameters. In the first case, it is possible to obtain high frequencies of oscillation of the exhaust titre about the set mean value which cannot, however, depart greatly from a stoichiometric titre. In the second case, it is possible to obtain a more significant polarisation of the titre, but the frequency of oscillation about the mean value decreases.
The object of the present invention is to provide a method for controlling the air/fuel ratio in an internal combustion engine which makes it possible to remedy the above-described drawbacks.
The present invention therefore relates to a method for controlling the titre of the exhaust gases in an internal combustion engine, in a control system comprising a control unit and oxygen sensor means, disposed along an exhaust duct of the engine and supplying a signal indicative of a titre of gases present in this exhaust duct, the control unit comprising memory means, controller means and correction and actuation means, these memory means receiving as input a plurality of engine-related parameters and an objective titre value and supplying as output a plurality of operating parameters, the controller means receiving as input the operating parameters and the signal and supplying as output a correction parameter correlated with the operating parameters and the signal, the correction and actuation means receiving as input a nominal quantity of fuel and the correction parameter and supplying as output to the engine an adjusted quantity of fuel, the method comprising the stages of:
a) selecting respective values of the operating parameters on the basis of the objective titre value and the engine-related parameters;
b) calculating the correction parameter on the basis of the respective values of the operating parameters and the signal;
c) calculating the adjusted quantity of fuel on the basis of the correction parameter and the nominal quantity of fuel;
the method being characterised in that the selection stage is preceded by the stages of:
d) selecting respective values of a first and a second of the operating parameters;
e) determining magnitudes as a function of these respective values of the first and the second operating parameters;
f) determining a third of the operating parameters as a function of the first and the second operating parameter and a mean value of the correction parameter.